Combustion control apparatus

ABSTRACT

This invention relates to a control circuit which controls combustion in a burner acting as a heat source of a hot-water boiler or a hot-air heater. More particularly, the present invention relates to a control circuit having a sequence which includes post-discharge such that electric discharge for ignition is still continued for a predetermined period of time, even after attainment of ignition. In a combustion control apparatus, when ignition is not attained within a predetermined safty period, the igniting operation is stopped by a safty timer for timing this safety period, thereby ensuring the safety. It is necessary to reliably discontinue the igniting operatin even when this safety timer is disabled. The present invention attains this object and ensures the safety with a reduced number of timers. That is, a timer for timing the post-discharge period is provided independently of the safety timer, these two timers starting their timing operations at the same time, so that the fuel valve can be cut off by an output from the post-discharge timer too in the event of failure of ignition.

DESCRIPTION TECHNICAL FIELD

This invention relates to a circuit in a combustion control apparatuswhich controls combustion in a burner acting as a heat source of ahot-water boiler or a hot-air heater. More particularly, the presentinvention relates to a control circuit having a sequence which includespost-discharge such that an electric discharge operation for ignitionmay still be continued even after attainment of ignition of fuel.

BACKGROUND ART

A combustion control apparatus must be constructed so as to ensuresafety by stopping the supply of fuel when fuel ignition is not attainedwithin a predetermined safety period of time after the ignitingoperation has been started. It must also be constructed so as to ensuresafety when a safety timer acting to stop the supply of fuel isdisabled.

A combustion control apparatus of this kind will be described withreference to FIGS. 1 and 2. In such an apparatus, pre-purge is effectedprior to starting of combustion, in which a combustion air blower isdriven to supply air for purging unburned gas remaining within afurnace. Description will be specifically directed to such an apparatusprovided with this pre-purging function. Description will also bespecifically directed to such an apparatus in which its operationsequence includes the step of post-discharge.

Referring to FIGS. 1 and 2, when the temperature of an object to beheated drops, a signal indicative of such a temperature drop is appliedthrough an input terminal 1 to a control circuit 2. This signalconstitutes an operation starting signal. In response to the applicationof this signal, a blower drive signal is delivered through a terminal 3,and a blower starts its operation to start pre-purge. At the same time,in response to the operation starting signal, a timer circuit 4 startsits timing operation by dividing the frequency of clock pulses appliedthrough a terminal 5. Upon lapse of a predetermined period of time(which will be referred to hereinafter as a pre-purge period), apre-purge termination signal is delivered from the timer circuit 4, andin response to this signal, an igniter drive signal and a fuel valvedrive signal are delivered through terminals 6 and 7 to drive an igniterand a fuel valve respectively, thereby commencing the ignitingoperation. The igniter effects electric discharge. When ignition isattained within a predetermined period of time (which will be referredto hereinafter as a safety period), a flame signal is applied to thecontrol circuit through a terminal 8 to continue the operation, and theelectric discharge is still continued for an additional predeterminedperiod of time (which will be referred to hereinafter as apost-discharge period). Thereafter, steady combustion is maintained.

If ignition is not sensed within the safety period, a judgement is madeso that there happens a trouble, and an ignition failure signal isgenerated in response to a safety period termination signal producedfrom the timer circuit 4, and a safety unit drive signal is deliveredthrough a terminal 9 so that a safety unit is operated to stop theblower, fuel valve and igniter, as shown by broken lines in FIG. 2, forensuring the safety.

Thus, when ignition fails to take place, the safety period terminationsignal appearing from the timer circuit 4 acts to cut off the fuelvalve. Therefore, if a trouble occurs in the timer timing the safetyperiod, the safety period termination signal would not be delivered, andfuel would be continuously discharged in the event of ignition failure,resulting in a serious danger.

In the prior art practice, two timers each timing this safety periodhave been provided to deal with such a dangerous situation. Therefore, atimer for timing the pre-purge period, a timer for timing thepost-discharge period and two timers for timing the safety period, thatis, a total of four timers have been necessarily required heretofore.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to ensure safety with a reducednumber of timers. According to the present invention, a safety timer fortiming the safety period and a post-discharge timer for timing thepost-discharge period are provided independently of each other, and thepost-discharge timer starts its timing operation in concurrent relationwith the starting of the timing operation of the safety timer to timethe safety period and then the post-discharge period follows thetermination of the safety period, so that an output from thepost-discharge timer can also act to cut off the fuel valve in the eventof ignition failure. According to the present invention, therefore, thefuel valve can be cut off upon termination of the post-discharge periodeven when the safety timer would not properly operate in the situationin which ignition fails to take place, and the post-discharge timerfunctions as a second safety timer for the safety timer, so that thesafety can be ensured with a reduced number of timers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a combustion control apparatus;

FIG. 2 is a diagram for explaining the operation of the individual unitsin the combustion control apparatus; and

FIG. 3 is a circuit diagram of an embodiment of the combustion controlapparatus according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described with reference to anembodiment shown in FIG. 3. This embodiment comprises an integratedcircuit having two sequences, one including post-discharge and the otherincluding no such a post-discharge. Signals at various terminals in FIG.3 are digital ones of "0" or "1".

Numerals 12 and 13 designate flip-flops constituting a reference timer14. Numeral 15 designates an AND gate, and numerals 16, 17 and 18designate flip-flops, which constitute a timer 19 (which will bereferred to hereinafter as a pre-purge timer) for timing the pre-purgeperiod. Numeral 20 designates an AND gate, and numerals 21 and 22designate flip-flops, which constitute another timer 23 (which will bereferred to hereinafter as a safety timer) for timing the safety period.Numeral 24 designates an AND gate, and numerals 25, 26 and 27 designateflip-flops, which constitute another timer 28 (which will be referred tohereinafter as a post-discharge timer) for timing the post-dischargeperiod.

Numeral 29 designates a terminal at which a signal of "1" instructingthe starting of operation appears in response to a signal indicative ofa temperature drop of an object to be heated through the terminal 1.This terminal 29 is connected through an inverter 30 to the resetterminals of the individual timers 14, 19, 23 and 28.

Numeral 31 designates a terminal which selectively indicates thepresence or absence of the post-discharge. A signal of "1" is applied tothis terminal 31 in the event of "presence". Numeral 32 designates aninverter, 33 a NAND gate and 34 an AND gate. An output terminal 35 ofthe AND gate 34 constitutes an igniter drive terminal, which isconnected to an output circuit having the terminal 6. Numeral 36designates an AND gate which is connected to a fuel valve drive terminal37. This terminal is connected to an output circuit having the terminal7. Numeral 38 designates a NAND gate, 39 an inverter and 40 an AND gate.The output terminal of this AND gate 40 constitutes a safety unit driveterminal 41 which is connected to an output circuit having the terminal9.

With such a construction, an explanation will be made of the case wherethe post-discharge is present. A signal of logic level "1" is applied tothe terminal 31. When the signal at the operation starting signalterminal 29 is turned from "0" to "1" to instruct the starting ofoperation, the blower is driven by a separate circuit to start thepre-purge, and at the same time, the output of the inverter 30 becomes"0" and the reset inputs of the individual timers 14, 19, 23 and 28 are"0" to release these timers from their reset state. Consequently, thereference timer 14 starts its frequency dividing operation on the clockpulses 5, and the pre-purge period begins from this time. When an outputQ₁₃ becomes "1", the output of the AND gate 15 becomes "1" since theoutput Q₁₈ of the pre-purge timer 19 is "1", and the pre-purge timer 19starts its frequency dividing operation.

Upon lapse of the pre-purge period, "1" is delivered at the output Q₁₈of the pre-purge timer 19. This output provides an ignition startingsignal. This signal Q₁₈ is applied to the AND gate 20 in the safetytimer 23 and to the AND gate 24 in the post-discharge timer 28, and,since the output Q₂₂ and the output Q₂₇ of these timers 23 and 28 are"1", these timers 23 and 28 start their frequency dividing operation inresponse to the output Q₁₃ of the reference timer 14. The safety periodbegins from this time. The level "0" at the output Q₁₈ of the pre-purgetimer 19 maintains the level "1" at the output Q₁₈.

The output of the inverter 32 is "0" at this time, and hence the outputof the NAND gate 33 is "1". Consequently, the output of the AND gate 34becomes "1" and thus the level at the igniter drive terminal 35 is "1"to drive the igniter, and the electric discharge is started in theigniter. The output Q₂₇ of the post-discharge timer 28 is "0" at thistime, and hence the output of the NAND gate 38 is "1". Since the inputsto the AND gate 36 are both "1", its output becomes "1" and thus thelevel at the fuel valve drive terminal 37 is "1" to open the fuel valve.As a result, the igniting operation starts.

When ignition is attained within the safety period and a flame signal of"1" is applied to the terminal 8, the output of the inverter 39 is "0",and the output of the AND gate 40 is "0" so that the level at the safetyunit drive terminal 41 remains "0". Therefore, the safety unit would notbe operated even if an output of "1" is delivered from the output Q₂₂ ofthe safety timer 23 upon lapse of the safety period. The electricdischarge in the igniter continues, since the output of the NAND gate 33remains "1", and hence the output of the AND gate 34 is maintained at"1". The output Q₂₂ of the safety timer 23 is "0", whereby the safetytimer 23 is held in that state.

As soon as the post-discharge period terminates with further lapse oftime, the output Q₂₇ of the post-discharge timer 28 becomes "0" to turnthe output of the AND gate 34 to "0", and the operation of the igniteris stopped. Although the output Q₂₇ becomes "1", the fuel valve ismaintained in its open position to permit a shift to the state of steadycombustion, since ignition has already been attained with the output ofthe NAND gate 38 being "1".

When the ignition is not attained within the safety period, the level atthe terminal 8 is "0". Thus, the output of the inverter 39 is "1" and,due to the output Q₂₂ of "1" of the safety timer 23, the output of theAND gate 40 becomes "1" and hence the level at the safety unit driveterminal 41 is "1". As a result, the safety unit is operated to cut offpower supply to the blower, igniter and fuel valve so as to stop theoperation and, at the same time, to display an alarm condition.

Further, when the ignition is not attained and upon lapse of thepost-discharge period, since the output Q₂₇ becomes "1", the inputs tothe NAND gate 38 are both "1", with the result that the output of theNAND gate 38 becomes "0" and the output of the AND gate 36 becomes "0".Thus, the level of the fuel valve drive terminal 37 is "0" to instructcutting-off of the fuel valve. The output Q₂₇ becomes "0" to turn theoutput of the AND gate 34 into "0" thereby instructing stoppage of theigniter. This state is held by the AND gate 24.

That is, in the event of failure of attainment of ignition, the fuelvalve is cut off in a duplicate manner through the safety unit by thesafety timer 23 and through the terminal 37 by the post-discharge timer28.

Therefore, even when a trouble is caused in any of the safety timer 23,the AND gate 40, the output circuit and the safety unit, and the alarmunit may not be operated in spite of failure of attainment of ignition,the output of the post-discharge timer 28 can close the fuel valve,thereby obviating the danger. That is, the post-discharge timer 28functions as a second safety timer for the safety timer 23.

Although the operation of the blower cannot be stopped in this case, theoperation of the blower is allowable since it is not dangerous in anyway. Further, although the alarm display unit such as an alarm lamp isalso not operated, the alarming condition can be recognized only by thecontinuous operation of the blower.

The above description has been directed to the case of failure ofattainment of ignition within the safety period, but similar proceduresare applicable to the case where flame extinction occurs after thesafety period. Upon lapse of the safety period, due to the output Q₂₂ of"0", the output Q₂₂ is maintained at "1". In the event of flameextinction within the post-discharge period, since the output Q₂₂ of thesafety timer is "1", the safety unit is operated as soon as the flameextinction occurs. And, upon lapse of the post-discharge period, theoutput circuit connected to the solenoid-operated valve is cut off. Inthe event of flame extinction after the post-discharge period, theoutput circuit connected to the solenoid-operated valve is cut offsimultaneously with the start of operation of the safety unit, since theoutput Q₂₂ and the output Q₂₇ are already "1".

Description will next be made of the case where the sequence does notinclude the post-discharge. In this case, the level at the terminal 31is "0". Therefore, the output from the NAND gate 33 is determined by theflame signal at the terminal 8. That is, when ignition is attained and"1" appears at the terminal 8, the output of the NAND gate 33 becomes"0" to stop the electric discharge in the igniter. The other operationsare similar to those of the case where the sequence includes thepost-discharge. Thus, the post-discharge timer 28 functions only as asecond safety timer for the safety timer 23. When the ignition fails tobe attained and the safety timer 23 is faulty, fuel will be continuouslydischarged until the fuel valve is cut off by the post-discharge timer,after the termination of the safety period. However, any problem willnot be caused if such a period of time is such that any danger will notbe given rise to within that period. Generally, the post-dischargeperiod in the burner of this kind is short, and therefore, thepost-discharge timer 28 can be utilized, as it is, as the second safetytimer.

In the aforementioned embodiment, the output of the NAND gate 38 is usedto control the fuel valve only, but it may be applied to the AND gate40. In such a case, the operation can be stopped by using the safetyunit, provided that the AND gate 40 is normal.

Although the aforementioned embodiment has been described with referenceto the case of including the pre-purge, it can also be applied to thecase where such a pre-purge is not included. Further, although thedescription has been made with reference to the case of involving twosequences, it can also be applied to the case where a single sequenceincluding post-discharge is involved.

In the aforementioned embodiment, the presence or absence of a flame ischecked after the safety period has elapsed. However, the output of theinverter 39 may be connected to the input of the AND gate 20 so that theoutput Q₂₂ may be delivered only when the flame signal is not applied.

In the aforementioned embodiment, it is described that the safety unitis operated in response to an occasional flame extinction. However, thesafety unit may be constructed so that it is not operated, if are-ignition operation or, for example, the start of pre-purge isrestored in response to an occasional flame extinction.

We claim:
 1. A combustion control apparatus, including fuel supply drivemeans and igniter drive means for allowing a fuel supply and an ignitionoperation to be effected at least for a first predetermined period oftime, said fuel supply being further continued, and said ignitionoperation being effected sequentially until termination of a secondpredetermined period of time subsequent to the first predeterminedperiod of time, when ignition is attained within the first predeterminedperiod of time, while said fuel supply and ignition operation beingdiscontinued at termination of the first predetermined period of time,when ignition is not attained within the first predetermined period oftime, said apparatus also including a first timer supplied with clockpulses for timing the first predetermined period of time and producingat the termination of the first predetermined period of time a signalindicating said discontinuation of the fuel supply and ignitionoperation when ignition is not attained within the first predeterminedperiod of time, and a second timer supplied with clock pulses for timingthe second predetermined period of time independently from the timingoperation of said first timer, wherein said apparatus further comprisesa first circuit generating a signal for cutting off a fuel valve inresponse to an output delivered from said second timer at thetermination of the second predetermined period of time and aflame-absence signal applied through a flame signal input terminal and asecond circuit for maintaining generation of said fuel valve cutting-offsignal.
 2. A combustion control apparatus, including fuel supply drivemeans for allowing a fuel supply to be effected at least for a firstpredetermined period of time, igniter drive means for allowing anignition operation to be effected at most for the first predeterminedperiod of time and a second predetermined period of time subsequent tothe first predetermined period of time, a first timer supplied withclock pulses for timing the first predetermined period of time andproducing at termination of the first predetermined period of time asignal indicating discontinuation of the fuel supply and ignitionoperation when ignition is not attained within the first predeterminedperiod of time, a second timer supplied with clock pulses for timing thesecond predetermined period of time independently from the timingoperation of said first timer, and selecting means for selecting, whenignition is attained within the first predetermined period of time, oneof continuation of the ignition operation until termination of thesecond predetermined period of time after the termination of the firstpredetermined period of time and discontinuation of the ignitionoperation within the first predetermined period of time, wherein saidapparatus further comprises a first circuit generating a signal forcutting off a fuel valve in response to an output delivered from saidsecond timer at the termination of the second predetermined period oftime and a flame-absence signal applied through a flame signal inputterminal and a second circuit for maintaining generation of said fuelvalve cutting-off signal, and said selecting means is connected to saidigniter drive means without affecting said second timer and firstcircuit so that said second timer delivers said output to said firstcircuit irrespectively of the selection of either said continuation orsaid discontinuation.
 3. A combustion control apparatus according toclaims 1 or 2, wherein said second circuit includes an output connectedto said second timer, an input to which the clock pulses to be suppliedto the second timer are applied, and another input supplied with asignal delivered from the second timer at the termination of the secondpredetermined period of time, thereby preventing supply of the clockpulses to said second timer after the termination of the secondpredetermined period of time.